Cells are too small to be seen without magnification.

Cells range in size from 1 to 100 micrometers. The study of cells, also called biology, would not have been possible without the invention of the microscope. With the advance microscopes of today such as the Scanning Electron Microscope and Transmission Electron Microscope, cell biologists are able to obtain detailed images of the smallest of cell structures.

Source: biology.about.com